Lets face it; most things in the natural world revolve around sex. Specifically, making sure you survive long enough to find and impress a mate—then have lots of offspring. It’s a complicated game that has led to all sorts of adaptations, but the one we’re sharing with you today is amongst the most surprising—the ability to change sex.

For lots of animals, sex is largely defined before birth, and depends on the chromosomes you receive during fertilisation. Yet for other animals like fruit flies, fish and some reptiles, things work a little bit differently. Depending on environmental and social conditions, some animals can change sex (and then even change back again!).

Here are six surprising animals that can change their sex.

6. Fish

Photo by Dezay / Shuttestock
Photo by Dezay / Shuttestock

Of all the sex changers in the animal kingdom, fish are the most well known. For clownfish, it’s very much a man’s world because all clownfish are, in fact, born male. The most dominant male of the group will become female. There will only be one female per clownfish group. If she dies, then it’s normally down to the largest male to change sex and take her place.  

Wrasses on the other hand work the other way, with groups being made of many females and one male. Exactly how this works is still something of a mystery, but it seems to consist of massive changes in hormone levels with ovaries transforming into testes. Amazingly, the fish can complete this transformation in as little as a week.

Whilst this was once thought to be rare, sex changes have now been observed in several dozen fish families. For many, it seems to be a strategy to ensure that the individual can find a mate. For example in the deep sea, where densities are extremely low, it may be very rare to encounter another of your species—thus being able to change sex would be a huge advantage.

5. Corals

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Forest fire mushroom coral. Photo by goobafish / Shutterstock

To jump to something a little more obscure, in 2008 it was discovered that mushroom corals can also change sex…in both directions! In comparison to fish, very little is known about the sex lives of corals, although it seems quite common for polyps to bud off from the parent in a form of clonal reproduction. Whether this ability to change sex will be discovered in other coral species remains to be seen.

4. Slugs

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The banana slug, so called because, you guessed it, it looks like a banana. Photo: Wikipedia

Amongst animals with a weird sex life, slugs are surely right up there at the top of the list. For slugs it seems choosing one sex just isn’t enough. Slugs are hermaphrodites, having both female and male reproductive organs, which they use to mate simultaneously.

Unfortunately, things get even weirder with a group called the banana slugs (Ariolimax). These slimy critters engage in Apophallation, which is the scientific term for biting off the partners’ penis. Some scientists have hypothesised that preventing the partner mating as a male again might be a selective advantage. Other than that, it’s just strange.

3. Frogs

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The Common reed frog. Photo: Wikipedia

Spontaneous sex change has also been observed in the common reed frog (Hyperolius viridiflavus) from West Africa (fans out there might also note that this is the same species of frog whose DNA was used to fill gaps in dinosaur DNA in the movie Jurassic Park).

Unfortunately in some cases human activity seems to also have had an impact on the sex of animals. A commonly used pesticide called atrazine has been found to change the sex of frogs exposed to it. It causes the frogs to increase production of oestrogen, turning males into fully functional females. The impact of this on wild populations is currently a hot research topic.

2. Snakes

The yellow bellied water snake. Photo by Nick Stroh / Shutterstock
The yellow-bellied water snake. Photo by Nick Stroh / Shutterstock

Who needs men anyway? Some female snakes such as the yellow-bellied water snake (Nerodia erythrogaster) kept alone in captivity have unexpectedly given birth to litters of baby snakes. Known as parthenogenesis, it’s a type of asexual reproduction and might have evolved as a last resort strategy when no males can be found.

Exactly how it works on a cellular level is still a mystery, but scientists think that under certain conditions one of the egg cells can behave like a sperm. More parthenogenesis has been described in other species such as sharks and amphibians, but so far it has never been reported in mammals.

1. Butterflies, Birds and Lobsters

Some birds can be both sexes simultaneously. A number of cardinals have been observed displaying both the red colouration of the male (right) on one half their body, and the more mottled grey colouration of the female (left) on the other. Photo by Bonnie Taylor Barry / Shutterstock
Some birds can be both sexes simultaneously. A number of cardinals have been observed displaying both the red colouration of the male (right) on one half their body, and the more mottled grey colouration of the female (left) on the other.
Photo by Bonnie Taylor Barry / Shutterstock

Last, but not least, very occasionally animals can be born as both male and female. But these unusual cases aren’t hermaphrodites, they are literally half of each sex. This is perhaps most striking in butterflies, where each wing could be a different colour. Known as gyandromorphs, it most likely occurs as a mistake in very early cell division. Gyandromorphs have subsequently been found in a handful of other animals, including birds and lobsters.

@James_Borrell